The increasing need for mobility in telecommunications systems requires the telephone network to cover larger geographical areas. A natural solution to increasing the geographical range of a telephone network is to expand the network. The expansion is optimized to suit the specified network standards. European Patent Application 0 602 857 discloses another solution for extending the range of a telephone network. The application illustrates how the services in a fixed telephone network can be reached transparently from a mobile unit located in a mobile telephone network. According to this publication, all outgoing calls from the mobile are first routed or directed to a program unit in the fixed telephone network. The call is then treated as though it had been setup from the program unit and access is given to the services of the fixed network. Another solution is disclosed in International Patent Application WO 94/13112, which illustrates a method of affording radio based access to a ground based telecommunications system. The aforesaid applications teach solutions for extending a telephone network having a specified signal standard, so as to cover a wider geographical area. If the solutions illustrated in these publications were to be applied to extend the coverage area of several networks that have mutually different specified standards, the total cost for each network proprietor would be far-reaching. U.S. Pat. No. 5,303,286 discloses a telecommunications system which includes PCN-system (Personal Communications Network) which has been integrated with a cellular network. The PCN-system is a part of the cellular network and when the PCN-system is heavily loaded certain frequencies in the cellular network can be allocated users in the PCN-system. The telecommunications system also includes a ground-based network part which is able to transfer signals from the PCN-user to the PCN-system via the cellular network, such that the user of the PCN-system is perceived as being present in the PCN-network despite the cellular network being used in the radio access. The two systems can be reached from the cellular network. A common radio unit is used for access to both systems. The solution proposed in the U.S. Patent Specification is complicated and fails to provide system flexibility and also requires a standard radio interface that has been specially adapted for the two systems that are included. The addition of further service networks having different types of specified signal standards would require further adaptations in addition to those already made and make the solution still more complicated.